The object of the invention is to be able to use an external system as a source of synchronization of a system using message-based synchronization. An external system typically uses a synchronization method different from that of a system utilizing message-based synchronization, but in principle, an external system can also use message-based synchronization, as will be described later.
As used in the text below, the term node refers to a junction point between transmission lines in a system. A node may be any device or equipment capable of affecting clock synchronization, such as a branching or cross-connection means.
Nodes in a system utilizing message-based synchronization are interconnected by transmission lines which the nodes use for data transmission. These lines also forward the clock frequency of the transmitting party to the receiving party. Each node selects the frequency of a signal from a neighbouring node, the frequency of its own internal clock source or a frequency applied to the node via a separate synchronization input from an external clock source as the source of its own clock frequency. In order that all nodes in the system would operate at the same clock frequency, one usually attempts to make the system synchronize itself with a single clock source called a master source. All system nodes connected directly to the selected master source are thus synchronized with the master source while nodes connected to the nodes adjacent to the master source but not directly connected to the master source are synchronized with these adjacent nodes. Accordingly, each node at a greater distance from the master source synchronizes itself with a node one node spacing closer to the master source.
In order that the above-described synchronization hierarchy could be established within the system, the system nodes interchange synchronization messages. These messages contain information by means of which individual nodes are able to select a timing source. The system nodes are prioritized and the system tends to synchronize itself with the clock frequency of a node having the highest level of priority. Normally each priority level is assigned to a single system node. Synchronization messages normally contain information about the origin of the clock frequency of the node transmitting the message and the priority of the node as well as a value describing the quality of the clock signal. Accordingly, a neighboring node clock frequency which originates from a desired node and which is of the highest quality can be selected by an individual node as the source of its own clock frequency. At the system start-up each node selects its own internal clock source as the source of its clock frequency as it has not yet processed any incoming synchronization messages. After the node has processed the first incoming synchronization messages, it selects the clock frequency of a neighboring node having the highest level of priority as the source of its clock frequency. After all messages have been distributed over the system and the system has achieved a stable state as far as synchronization is concerned, the system has been synchronized hierarchically with the clock frequency of the master source.
FIG. 1 shows a system MS utilizing message-based synchronization in a stable situation. Priorities assigned to the nodes are indicated by numbers within the circles representing the nodes. The smaller the number, the higher the priority of the node. Synchronization messages transmitted by a node n (n=1 . . . 6) are indicated by the reference MSGn. Synchronization messages transmitted by different nodes usually differ from each other and depend on the applied message-based synchronization method. The distribution of the clock frequency from the master clock (node 1) to the other system nodes is illustrated by solid lines. Internodal connections drawn by broken lines are not used in a normal situation for system synchronization, but they are available in change situations.
Message-based synchronization is based on a simple principle that the user defines the synchronization hierarchy of the nodes by assigning each node a dedicated signature indicating the hierarchical level of the node and that the system synchronizes itself with the defined master clock independently by utilizing, if required, all existing internodal connections. If the connection to the master clock fails, and no alternative connection exists, or if the master clock fails, the system synchronizes itself with a node of the next highest level of hierarchy. Response to a change in synchronization takes place by message interchange between nodes.
Message-based synchronization methods of the type described above are described e.g. in U.S. Pat. Nos. 2,986,723 and 4,837,850, which are referred to for a more detailed description. Messages used in one prior art message-based synchronization method (SOMS) will be described more closely below with reference to FIGS. 2 and 3.
A system employing message-based synchronization forms a kind of closed system, at least as far as synchronization is concerned (often also with respect to data transmission). This is problematic as there nevertheless exists a need to interconnect different types of networks.
In the above-mentioned U.S. Pat. No. 4,837,850, an exchange of a network utilizing message-based synchronization is connected to an exchange of a higher level, but fixed definitions are used in the network in such a manner that all those exchanges that are connected to such an exchange have the highest classification within the network. An arrangement of this type does not enable, for instance, several external sources of synchronization to be connected flexibly with the system.